1. Introduction
This invention relates to polymer blends and more particularly, to blends of a polyphenylene ether and a polystyrene stabilized with a stabilizer combination comprising an inorganic sulfide alone or in combination with an organic phosphite.
2. Description of the Prior Art
The polyphenylene ethers are known and described in numerous publications including U.S. Pat. Nos. 3,306,874 and 3,306,875 of Allan S. Hay and U.S. Pat. Nos. 3,257,357 and 3,257,358 of Gelu Stoeff Stamatoff, all incorporated herein by reference. The high molecular weight polyphenylene ethers are high performance engineering thermoplastics possessing relatively high melt viscosities and softening points; i.e., in excess of 250.degree. C., and are useful for many commercial applications requiring high temperature resistance including formation of film, fiber and molded articles.
In U.S. Pat. No. 3,383,435, a polymer blend is disclosed comprising a polyphenylene ether and a polystyrene. The polystyrene improves the melt processability of the polyphenylene ether while the polyphenylene ether simultaneously upgrades many of the properties of the polystyrene. The invention of this patent is based upon the discovery that the polyphenylene ethers and the polystyrenes, including the modified polystyrenes, are combinable in all proportions resulting in blends having many properties improved over those of either of the components. As disclosed in said patent, the polystyrenes combinable with the polyphenylene ether are those having at least 25% by weight polymer units derived from a monomer having the formula: ##STR1## where R is a hydrogen, lower alkyl or halogen; Z is a member selected from the class consisting of vinyl, hydrogen, halogen and lower alkyl; and p is 0 or whole number equal to from 1 to 5. The preferred composition of this patent is a poly-(2,6-dialkyl-1,4-phenylene)ether combined with polystyrene or rubber-modified polystyrene.
It is known in the art that both the polyphenylene ethers and the polystyrenes are somewhat unstable under the influence of heat and light in oxygen containing atmospheres causing the polymers to become dark colored, brittle and undesirable for many uses. As a result, many effective stabilizing systems have been found for each of the polymers alone. However, it has also been found that using combinations of known stabilizers for the polyphenylene ethers and the polystyrenes does not provide optimum stabilization of blends of these polymers.